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From:   Deepak Gupta <debug@rivosinc.com>
Date:   Fri, 17 Mar 2023 12:26:08 -0700
Message-ID: <CAKC1njQL8zNHh35wbpQ71ncqOTdm-L9+sbp9aDmUOHRcqV+Z6A@mail.gmail.com>
Subject: Re: [PATCH v7 22/41] mm/mmap: Add shadow stack pages to memory accounting
To:     "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc:     "Hansen, Dave" <dave.hansen@intel.com>,
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On Fri, Mar 17, 2023 at 10:42=E2=80=AFAM Edgecombe, Rick P
<rick.p.edgecombe@intel.com> wrote:
>
> On Fri, 2023-03-17 at 10:28 -0700, Deepak Gupta wrote:
> > On Fri, Mar 17, 2023 at 10:16=E2=80=AFAM Dave Hansen <dave.hansen@intel=
.com>
> > wrote:
> > >
> > > On 3/17/23 10:12, Deepak Gupta wrote:
> > > > >   /*
> > > > > - * Stack area - automatically grows in one direction
> > > > > + * Stack area
> > > > >    *
> > > > > - * VM_GROWSUP / VM_GROWSDOWN VMAs are always private
> > > > > anonymous:
> > > > > - * do_mmap() forbids all other combinations.
> > > > > + * VM_GROWSUP, VM_GROWSDOWN VMAs are always private
> > > > > + * anonymous. do_mmap() forbids all other combinations.
> > > > >    */
> > > > >   static inline bool is_stack_mapping(vm_flags_t flags)
> > > > >   {
> > > > > -       return (flags & VM_STACK) =3D=3D VM_STACK;
> > > > > +       return ((flags & VM_STACK) =3D=3D VM_STACK) || (flags &
> > > > > VM_SHADOW_STACK);
> > > >
> > > > Same comment here. `VM_SHADOW_STACK` is an x86 specific way of
> > > > encoding a shadow stack.
> > > > Instead let's have a proxy here which allows architectures to
> > > > have
> > > > their own encodings to represent a shadow stack.
> > >
> > > This doesn't _preclude_ another architecture from coming along and
> > > doing
> > > that, right?  I'd just prefer that shadow stack architecture #2
> > > comes
> > > along and refactors this in precisely the way _they_ need it.
> >
> > There are two issues here
> >  - Encoding of shadow stack: Another arch can choose different
> > encoding.
> >    And yes, another architecture can come in and re-factor it. But so
> > much thought and work has been given to x86 implementation to keep
> >    shadow stack to not impact arch agnostic parts of the kernel. So
> > why creep it in here.
> >
> > - VM_SHADOW_STACK is coming out of the VM_HIGH_ARCH_XX bit position
> > which makes it arch specific.
> >
> >
>
> VM_SHADOW_STACK is defined like this (trimmed for clarity):
> #ifdef CONFIG_X86_USER_SHADOW_STACK
> # define VM_SHADOW_STACK        VM_HIGH_ARCH_5
> #else
> # define VM_SHADOW_STACK        VM_NONE
> #endif

Ok.

>
> Also, we actually had an is_shadow_stack_mapping(vma) in the past, but
> it was dropped from other feedback.

looks like I've been late to the party.
IMHO, that was the right approach.

>
>